Most recent vehicles are now equipped with keyless entry systems, which make it possible to lock and unlock the doors of a vehicle by operating a portable machine (a remote operation key) without needing to lock/unlock the mechanical locks of the doors. Such remote operations often use ultra-high frequency waves (UHF), which have a short propagation distance and are weak, and the portable machine transmits a wireless signal obtained by modulating a carrier wave with information including identification information set for each particular vehicle. A vehicle-mounted machine that has received the wireless signal from the portable machine demodulates the wireless signal, verifies the obtained identification information against stored identification information, and locks/unlocks a door if there is a match.
In addition to locking/unlocking doors, some keyless entry systems can control vehicle-mounted devices aside from the doors, depending on the state of the vehicle-mounted device and the content of the wireless signal from the portable machine.
For example, JP-10-131569A discloses a keyless entry system that raises/lowers power windows in the case where the operation of an operation switch of a portable machine is started within a first amount of time after all doors have been locked/unlocked and the operation then continues for more than a second amount of time.
The portable machine of this keyless entry system transmits a wireless signal including a header, a portable machine-unique ID (identification information), a control code of the operation switch (control information), and a continuous operation code indicating the operation time of the operation switch, in that order. The continuous operation code is transmitted continuously as long as the continuous operation time of the operation switch does not exceed a limit.
However, with the technique disclosed in JP-10-131569A, other wireless signals transmitted between other portable machines and vehicle-mounted machines, external noise such as electromagnetic pulses, and the like may intermix with the wireless signal transmitted from the portable machine to the vehicle-mounted machine and temporarily interrupt the reception of the wireless signal by the vehicle-mounted machine. In such a case, it is difficult for the vehicle-mounted machine to continue the reception. Specifically, once the reception is resumed, the received wireless signal will not contain the ID, and thus the vehicle-mounted machine will ignore that wireless signal. There is thus a problem in that the reception of a wireless signal cannot be continued if that wireless signal has been interrupted.
Having been achieved in light of such circumstances, it is an object of the present invention to provide a vehicle-mounted communication system with which a vehicle-mounted machine is able to continue receiving a wireless signal even if the signal is interrupted while the vehicle-mounted machine is receiving the wireless signal from a portable machine, as well as the portable machine and the vehicle-mounted machine constituting the vehicle-mounted communication system.